Host-Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

Probing substrate binding inside a paramagnetic cavity: a NMR spectroscopy toolbox for combined experimental and theoretical investigation

Protein cavities often rely on the paramagnetic metal present in their active site in order to catalyse various chemical transformations in biology. The selective detection and identification of the substrate is of fundamental importance in environmental monitoring and biological studies. Herein, a covalently linked Fe(iii)porphyrin dimer-based paramagnetic sensory cavity has been devised for the accurate detection and simultaneous identification of phenol (substrate) binding within the cavity that provides a unique spectroscopic signature with valuable structural and environmental information. These substrates within the paramagnetic cavity leave the fingerprints of the specific binding modes (exo vs. endo) which are well distinguished with the help of various spectroscopic studies viz. UV-vis, 1H, and 19F NMR and in their respective crystal structures also. The theoretical 19F NMR analysis plays a pivotal role in replicating the observed NMR trends with large chemical shifts of the phenolato species which in turn helps in deciphering the selective binding modes of the phenols and thereby recognizing the chemical environment within the cavity. These findings will help develop an excellent diagnostic tool for in situ monitoring of subtle conformational changes and transient interactions.

Kinetics of ligand exchange in solution: a quantitative mass spectrometry approach

Complex speciation and exchange kinetics of labile ligands are critical parameters for understanding the reactivity of metal complexes in solution. We present a novel approach to determine ligand exchange parameters based on electrospray ionization mass spectrometry (ESI-MS). The introduction of isotopically labelled ligands to a solution of metal host and unlabelled ligands allows the quantitative investigation of the solution-phase equilibria. Furthermore, ion mobility separation can target individual isomers, such as ligands bound at specific sites. As a proof of concept, we investigate the solution equilibria of labile pyridine ligands coordinated in the cavity of macrocyclic porphyrin cage complexes bearing diamagnetic or paramagnetic metal centres. The effects of solvent, porphyrin coordination sphere, transition metal, and counterion on ligand dissociation are discussed. Rate constants and activation parameters for ligand dissociation in the solution can be derived from our ESI-MS approach, thereby providing mechanistic insights that are not easily obtained from traditional solution-phase techniques.

Dynamic and Persistent Cyclochirality in Hydrogen-Bonded Derivatives of Medium-Ring Triamines

Cyclic triureas derived from 1,4,7-triazacyclononane (TACN) were synthesized; X-ray crystallography showed a chiral bowl-like conformation with each urea hydrogen-bonded to its neighbor with uniform directionality, forming a "cyclochiral" closed loop of hydrogen bonds. Variable-temperature 1H NMR, 1H-1H exchange spectroscopy, Eyring analysis, computational modeling, and studies in various solvents revealed that cyclochirality is dynamic (ΔG‡25°C = 63-71 kJ mol-1 in noncoordinating solvents), exchanging between enantiomers by two mechanisms: bowl inversion and directionality reversal, with the former subject to a slightly smaller enantiomerization barrier. The enantiomerization rate substantially increased in the presence of hydrogen-bonding solvents. Population of only one of the two cyclochiral hydrogen-bond directionalities could be induced by annulating one ethylene bridge with a trans-cyclohexane. Alternatively, enantiomerization could be inhibited by annulating one ethylene bridge with a cis-cyclohexane (preventing bowl inversion) and replacing one urea function with a formamide (preventing directionality reversal). Combining these structural modifications resulted in an enantiomerization barrier of ΔG‡25°C = 93 kJ mol-1, furnishing a planar-chiral, atropisomeric bowl-shaped structure whose stereochemical stability arises solely from its hydrogen-bonding network.

Construction and Hierarchical Self-Assembly of Multifunctional Coordination Cages with Triangular Metal-Metal-Bonded Units

Herein, a series of face-capped (Tr₂M₃)₄L₄ (Tr = cycloheptatrienyl cationic ring; M = metal; L = organosulfur ligand) tetrahedral cages 1-3 functionalized with 12 appended crown ether moieties were designed and synthesized. The reversible binding of ammonium cations with peripheral crown ether moieties to adjust internal guest-binding was realized. Combination of a bisammonium linker and cage 3 led to the formation of a supramolecular gel SPN1 via host-guest interactions between the crown ether moieties and ammonium salts. The obtained supramolecular gel exhibited multiple-stimuli responsiveness, injectability, and excellent self-healing properties and could be further developed to a SPN1-based drug delivery system. In addition, the storage modulus of SPN1 was 20 times higher than that of the model gel without Pd-Pd bonded blocks, and SPN1 had better self-healing properties compared with the latter, demonstrating the importance of such cages in improving mechanical strength without losing the dynamic properties of the material. The cytotoxicity in vitro of the drug-loaded (doxorubicin or methotrexate) SPN1 was significantly improved compared to that of free drugs.

113Cd as a Probe in NMR Studies of Allosteric Host-Guest-Ligand Complexes of Porphyrin Cage Compounds

Cadmium porphyrin cage compounds Cd1 and 113 Cd1 have been synthesized from the free base porphyrin cage derivative H21 and Cd(OAc)2 ⋅ 2 H2O or 113Cd(OAc)2 ⋅ 2 H2O, respectively. The compounds form allosteric complexes with the positively charged guests N,N'-dimethylimidazolium hexafluorophosphate (DMI) and N,N'-dimethylviologen dihexafluorophosphate (Me2V), which bind in the cavity of the cage, and tbupy, which coordinates as an axial ligand to the outside of the cage. In the presence of tbupy, the binding of DMI in Cd1 is enhanced by a factor of ∼31, while the presence of DMI or Me2V in the cavity of Cd1 enhances the binding of tbupy by factors of 55 and 85, respectively. The X-ray structures of the coordination complexes of Cd1 with acetone, acetonitrile, and pyridine, the host-guest complex of Cd1 with a bound viologen guest, and the ternary allosteric complex of Cd1 with a bound DMI guest and a coordinated tbupy ligand, were solved. These structures revealed relocations of the cadmium center in and out of the porphyrin plane, depending on whether a guest or a ligand is present. 113Cd NMR could be employed as a tool to quantify the binding of guests and ligands to 113 Cd1. 1D EXSY experiments on the ternary allosteric system Cd1-tbupy-Me2V revealed that the coordination of tbupy significantly slowed down the dissociation of the Me2V guest. Eyring plots of the dissociation process revealed that this kinetic allosteric effect is entropic in nature.

Paramagnetic relaxation enhancement NMR as a tool to probe guest binding and exchange in metallohosts

Paramagnetic metallohost systems can bind guest molecules and find application as biomimetic catalysts. Due to the presence of the paramagnetic metal center, rigorous characterization of these systems by NMR spectroscopy can be very difficult. We report here that metallohost-guest systems can be studied by using the paramagnetic relaxation enhancement (PRE) effect. Manganese(III) porphyrin cage compounds are shown through their PRE to thread and bind viologen guests, including a polymeric one. The binding constants and dethreading activation parameters are lower than those of the metal-free porphyrin cage compounds, which is proposed to be a result of charge repulsion of the trivalent metal center and dicationic viologen guest. The threading rate of the manganese(III) porphyrin cage onto the polymer is more than 10 times faster than that of the non-metallated one, which is ascribed to initial binding of the cage to the polymer chain prior to threading, and to an entron effect.

Paramagnetic metallohost systems are difficult to characterize. Here the authors report that the paramagnetic relaxation enhancement effect can be used to prove by nuclear magnetic resonance experiments that Mn(III) porphyrin cage compounds can bind and thread low molecular weight and polymeric guests.

Internal and External Catalysis in Boronic Ester Networks

In dynamic materials, the reversible condensation between boronic acids and diols provides adaptability, self-healing ability, and responsiveness to small molecules and pH. The thermodynamics and kinetics of bond exchange determine the mechanical properties of dynamic polymer networks. Here, we investigate the effects of diol structure and salt additives on the rate of boronic acid-diol bond exchange, binding affinity, and the mechanical properties of the corresponding polymer networks. We find that proximal amides used to conjugate diols to polymers and buffering anions induce significant rate acceleration, consistent with an internal and external catalysis, respectively. This rate acceleration is reflected in the stress relaxation of the gels. These findings contribute to the fundamental understanding of the boronic ester dynamic bond and offer molecular strategies to tune the macromolecular properties of dynamic materials.

Interactions between PAMAM-NH2 and 6-Mercaptopurine: Qualitative and Quantitative NMR studies

Despite being used as an anti-leukemic drug, the poor solubility of 6-mercaptopurine (6-MP) limits its use in topical and parenteral applications. Dendrimers are commonly used as drug carriers to improve their solubility in aqueous solution. In this work, the interactions between 6-MP and the amine-terminated poly(amidoamine) dendrimers (PAMAM-NH₂ ) were investigated by various NMR technology. The chemical shift titrations disclosed that the 6-MP interacted with the surface of PAMAM-NH₂ mainly through electrostatics. The determination of diffusion coefficient and relaxation measurements further confirmed the presence of interactions in 6-MP/PAMAM-NH₂ complexes. In addition, the encapsulation of 6-MP within the cavity of PAMAM-NH₂ was revealed through nuclear Overhauser effect spectroscopy and Saturation Transfer Double Difference analysis. Finally, the binding strength (H-8 is 100% and H-2 is 70%) of 6-MP to PAMAM-NH₂ was quantitatively expressed using epitope maps. This study provides a systematic methodology for qualitative and quantitative studies of the interactions between dendrimers and drug molecules in general.

Host-Guest Exchange of Viologen Guests in Porphyrin Cage Compounds as Studied by Selective Exchange Spectroscopy (1D EXSY) NMR

Dynamics in complexes of porphyrin cage compounds and viologen-derived guest molecules are investigated by selective exchange NMR spectroscopy (1D EXSY). Exchange rates were found to be independent of excess guest concentration, revealing a dissociative exchange mechanism, which is accompanied by negative activation entropies, indicating significant reorganization of the host-guest complex during dissociation. Nonsymmetric viologen guests with bulky head groups had more unidirectional binding and slower exchange rates than guests with less-bulky head groups. Thermodynamic and kinetic studies revealed that the exchange process is primarily driven by the thermodynamics of binding and that guest binding can be influenced by introducing steric and electronic groups on the host . Exchange studies with guests bearing a polymer chain revealed that both slippage and full dissociation takes place and the rate constants for both processes were determined. The slippage rate constant revealed that for smaller guests exchange takes place nearly exclusively under thermodynamic control.